1. Field of the Invention
The present invention relates to a balun for transforming a balanced signal to an unbalanced signal and vice versa.
2. Description of the Related Art
A balun (balanced-unbalanced transformer) is a device for transforming a balanced signal to an unbalanced signal and vice versa. Here, an unbalanced signal is a signal of which reference potential is the ground potential. A balanced signal is made up of two signals that are nearly 180 degrees out of phase with each other and have nearly equal amplitudes. The balun has two balanced terminals for receiving and outputting balanced signals and one unbalanced terminal for receiving and outputting unbalanced signals.
Baluns are used in mobile communications devices including cellular phones and communications devices for radio local area network (LAN). Reductions in size and thickness of the mobile communications devices have been strongly sought, and techniques for mounting components with higher density have been therefore required. Integration of components through the use of a multi-layer substrate has been thus proposed. The multi-layer substrate is a layered structure including dielectric layers and patterned conductor layers alternately stacked.
Many of conventional baluns that have been widely used are those having at least two pairs of quarter-wave strip lines electromagnetically coupled to each other (which are called electromagnetic-coupling-type baluns in the present patent application). Various sorts of multi-layer electromagnetic-coupling-type baluns made by using multi-layer substrates have been proposed, too, as disclosed in the Published Unexamined Japanese Patent Application Heisei 9-260145 (1997), the Published Unexamined Japanese Patent Application 2000-188218, and the Published Unexamined Japanese Patent Application 2002-190413, for example.
It is required for the electromagnetic-coupling-type baluns that a higher degree of coupling be achieved between a pair of quarter-wave strip lines. However, a problem of multi-layer electromagnetic-coupling-type baluns is that it is difficult to increase the coupling between a pair of quarter-wave strip lines, and that insertion loss is thereby increased.
Regarding this problem, the Published Unexamined Japanese Patent Application Heisei 10-200360 (1998) discloses a balun using a high-pass filter and a low-pass filter (which is called a filter-type balun in the present patent application) as a balun that is capable of reducing insertion loss even if formed to have a multi-layer structure.
In the filter-type balun disclosed in the Published Unexamined Japanese Patent Application Heisei 10-200360, the high-pass filter and the low-pass filter is each made up of an LC circuit comprising a single coil and a single capacitor. This filter-type balun is capable of making insertion loss smaller, compared with the electromagnetic-coupling-type balun. However, the filter-type balun has a problem that, in a neighborhood of the frequency band in which the balun is used, the difference in amplitude between output signals at the two balanced terminals greatly varies, depending on frequencies, and this narrows the frequency range in which the difference in amplitude of output signals at the two balanced terminals satisfies a desired standard.
A multi-layer filter-type balun achieves a reduction in size of the balun. However, electromagnetic interference often occurs among a plurality of coils of the multi-layer balun. Such interference causes a difference between desired characteristics of the balun as designed and characteristics of the balun actually fabricated. It is therefore difficult to fabricate the balun that has the desired characteristics.